Multiple input multiple output (MIMO) systems utilize multiple transmit and receive antennas to offer various improvements over traditional single input single output (SISO) wireless communication systems, such as increases in throughput and range at the same bandwidth and same overall transmit power expenditure. In general, MIMO technology can increase the spectral efficiency of a wireless communication system. Wireless MIMO communication exploits phenomena such as multipath propagation to potentially increase data throughput and range, or reduce error rates, rather than attempting to eliminate the effects of multipath propagation, as traditional SISO communication systems often seek to do.
A fractal is a geometric shape that can be subdivided in parts, each of which is (at least approximately) similar to the whole. Fractals generally have three properties: (a) self-similarity; (b) a fractal or Hausdorff dimension (usually greater than the shape's topological dimension); and (c) production by an iterative process. A “true” fractal features self-similarity at all resolutions and is generated by infinite iterations. In reality, fractal shapes are generated by a finite process iterated a finite number of times. However, such finite fractal-like shapes may comprise sufficient approximations such that they may be referred to, and considered, as fractals. Some of the more well-known fractals include the Koch snowflake, Sierpinski triangle, Cantor set, Julia set and Mandelbrot set.
U.S. Pat. No. 6,140,975 to Cohen shows various fractal antenna structures. For example, FIG. 7D-5 shows an antenna system having fractal ground elements and a fractal vertical element. Note that these antenna elements must be placed orthogonally and are then tuned by varying the separation distance or by forming a “cut” in an element. FIG. 8B shows an arrangement of fractal antennas which form a sectorized antenna array. A circuit selects the element having the best orientation toward the base station (e.g., by determining the strongest signal). Thus, only one of the antenna elements is active at any given time. The antenna elements are preferably fed for vertical polarization. Furthermore, the antenna system must be tuned for each particular conformal shape. U.S. Pat. No. 6,452,553 to Cohen describes additional fractal antenna structures.